This invention is directed to contour forming machines for contour forming sheet metal structural elements and more particularly to apparatus useful in systems for controlling the operation of contour forming machines.
Contour forming machines for forming contours in sheet metal structural elements or parts, such as body frames, stiffeners and stringers used in aircraft, for example, fall generally into two classes -- stretch forming machines and contour rolling machines.
Stretch forming machines cause the parts to yield under tension while being maintained in a plastic state. The parts are wrapped against a form block of the desired contour until the desired part contour is achieved. Because an individual form block is required for each part, stretch forming has the disadvantage of requiring considerable lead time for form block fabrication and testing. With most high-strength materials, springback factors must be experimentally developed and incorporated into the form block contour. In addition to the expense attendant to developing individual form blocks, stretch forming has the further disadvantage of requiring the maintainence and storage of an expensive inventory of large, fairly complex tools. Moreover, stretch forming has the mechanical disadvantage that when a part is formed in tension it has a strong tendency to thin and neck, thus losing structural strength. In order to avoid this tendency, filler blocks and a variety of elongation control devices are frequently employed all of which further increase the cost of contour forming using this process.
In roll forming machines a part is traversed through an arrangement of rolls that are progressively offset to impart increasing contour. The operator of the roll forming machine continues to increase the contour of the part by adjustment of the rolls until he determines through frequent reference to a template or checking fixture that the desired contour configuration has been achieved. Because only a small increment of the part is formed at a time, and because the geometric arrangement of the rolls provide a considerable leverage factor, roll forming facilities are smaller than comparable stretch forming facilities. Moreover, roll forming machines are considerably less expensive than corresponding stretch forming machines. However, classically, only uniplanar contours, as opposed to multiplanar contours, have been formed using roll forming facilities. Moreover, in the past it has been found that parts with complex cross-sections cannot be readily contour formed. Further, manual adjustment of roll positions in accordance with the operators experience is necessary to compensate for varying part springback. Because of these disadvantages, the time required to form a particular part and the resulting part quality depend to a large extent on the skill of the operator. Because operator skill is involved part forming costs rapidly increase with critical tolerance requirements. On the other hand, only minor distortion of the cross-section configuration of a part occurs during roll forming because only a small portion of the part is unsupported at any one time between the forming rolls.
Generally speaking, as can be seen from the foregoing discussion, stretch forming facilities are more suitable for large scale production because they require less operator skill, even though an expensive inventory of fairly complex tooling is required. Roll forming facilities, on the other hand, are better adapted to prototype development and special situations because labor costs are considerably higher due to the high degree of operator skill required.
Recently a numerically controlled contour forming machine has been developed. This machine is described in a Defense Documentation Center (DDC) document entitled "Numerically Controlled Contour Forming" by G. B. Foster. This document is a copy of Interim Technical Report IR-791-0 (I) submitted by the Commercial Airplane Group, The Boeing Company under Air Force contract F 33615-70-C-1827. The document bears the identification number D6-22258. The information contained in this document is incorporated herein by reference for background information purposes. In general, this document describes a control system for controlling the positioning of the right and left forming rolls, which are located on either side of the pinch rolls between which a part to be contour formed passes, of a roll forming machine. The roll forming machine can create complex contours as well as simple (uniplanar) contours in parts with complex as well as simple cross-sectional configurations.
One of the problems with roll forming machines, whether they be manual or electronically controlled, is that springback of the part occurs after it passes through the contour forming rolls, and it is necessary to adjust the rolls to compensate for such springback to obtain the desired contour. The apparatus disclosed in the document referenced above includes a uniplanar sensor for sensing springback in a single plane and using this information to control the positioning of the forming rolls. The document does not, however, disclose a multiplanar sensor for sensing springback even though the apparatus described in the document includes the capability of multiplanar contour forming. This invention is directed to such a multiplanar sensor and a control system for using the information developed by the sensor to control a roll forming machine in order for it to compensate for springback.
Therefore, it is an object of this invention to provide a multiplanar contour sensor.
It is a further object of this invention to provide a multiplanar contour sensor and a control system for modifying the signals applied to a numerically controlled roll forming machine in accordance with information derived by the multiplanar sensor.
It is a still further object of this invention to provide a control system for use with a numerically controlled roll forming machine which includes a sensor for sensing the contour in a multitude of planes of a part being formed by the machine and a means for modifying the signals applied to the forming rolls of the machine in accordance with the information sensed by the multiplanar sensor.